Graft-vs.-host reactions were used to detect phenotypic differences between genotypically identical reciprocal hybrid mice. Cells from either postpartum or virgin female mice of the two parental strains were injected into newborn reciprocal hybrid recipients. With all donors CBL recipients (BALB/c female X C57B male) gave greater spleen indices than did BLC's (C57B1 female X BALB male). Post partum BALB/c's that had borne CBL litters appeared to be immunized to antigens unique to CBLs. Post partum BALB's also showed more rapid graft rejection of skin from CBL as compared to BLC. There was no difference in the ability of CBL and BLC cells to stimulate in MLC. These results are interpreted as relecting a maternal modification of fetal antigens.